## What changes were proposed in this pull request?
The motivation for these additional metrics is to help in troubleshooting and monitoring task execution workload when running on a cluster. Currently available metrics include executor threadpool metrics for task completed and for active tasks. The addition of threadpool taskStarted metric will allow for example to collect info on the (approximate) number of failed tasks by computing the difference thread started – (active threads + completed tasks and/or successfully finished tasks).
The proposed metric finishedTasks is also intended for this type of troubleshooting. The difference between finshedTasks and threadpool.completeTasks, is that the latter is a (dropwizard library) gauge taken from the threadpool, while the former is a (dropwizard) counter computed in the [[Executor]] class, when a task successfully finishes, together with several other task metrics counters.
Note, there are similarities with some of the metrics introduced in SPARK-24398, however there are key differences, coming from the fact that this PR concerns the executor source, therefore providing metric values per executor + metric values do not require to pass through the listerner bus in this case.
## How was this patch tested?
Manually tested on a YARN cluster
Closes#22290 from LucaCanali/AddMetricExecutorStartedTasks.
Lead-authored-by: Luca Canali <luca.canali@cern.ch>
Co-authored-by: LucaCanali <luca.canali@cern.ch>
Signed-off-by: Marcelo Vanzin <vanzin@cloudera.com>
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